The invention applies most particularly to underwater vehicles comprising a vectored thruster with two propellers. A thruster is termed vectored when it can be controlled so as to produce a thrust or propulsion force orientable over 4π steradians. So-called vectored propulsion of an underwater vehicle is opposed to conventional propulsion in which the orientation of control surfaces brings about a modification of the lift generated by the flow of fluid surrounding the control surfaces. The force generated by the fluid on the control surfaces makes it possible to orient the vehicle in the sought-after direction. A well-known limit of this form of propulsion is the need to generate an appreciable fluid flow around the vehicle in order to bring about an alteration in lift of the control surfaces allowing a change of attitude of the vehicle, that is to say in order to make it possible to maneuver the underwater vehicle. If this flow is too weak then the effectiveness of the control surfaces decreases inversely as the square of the speed of the flow until it becomes zero for a zero flow speed. Stated otherwise, it is not possible by conventional propulsion to orient the vehicle in a sought-after direction without an appreciable displacement of the vehicle, when the fluid flow is zero. Moreover the control surfaces generate a drag proportional to the square of the speed which opposes the displacement and which therefore consumes energy, the more so the more the control surfaces are invoked. The method for controlling a vectored propulsion presented in the present patent allows the vehicle to do away with conventional (rudder) control surfaces, and therefore makes it possible to appreciably reduce the hydrodynamic drag of the vehicle. Vectored propulsion of the type with two propellers presents numerous theoretical advantages, notably enhanced mobility, simplification of the architecture (e.g. by dispensing with the control surfaces), increase in the endurance of the vehicle (by reducing the hydrodynamic drag). This absence of any control surface other than the blades of the propellers facilitates the realization of a so-called “flush” hydrodynamic vehicle, that is to say from which no appendage protrudes, thereby allowing it for example to fit easily in a tube and avoiding damaging the control surfaces when docking alongside.
However, the controlling of a thruster with two propellers encounters numerous difficulties notably at low speed.